The present invention pertains generally to injectors for subcutaneous delivery of a fluid medicament to a patient. More particularly, the present invention pertains to so-called needleless injectors. The present invention is particularly, but not exclusively, useful as an injector which will stabilize the tip of an injection tube against the skin of the patient to maintain a reliable skin-to-syringe relationship during an injection of fluid medicament.
For many years, so-called needleless injectors have been commercially available for use in delivering fluid medicaments to a patient. The interest in this type of injector has been generated primarily for safety purposes because needleless injectors typically do not have the sharp projections which can accidentally or inadvertently cause unwanted cuts or punctures. More recently, the interest in needleless injectors has been heightened by the knowledge that many serious illnesses, such as AIDS, can be transmitted by needle punctures. Although otherwise desirable, the design of a needleless injector requires the balancing of often competing concerns which, if not properly handled, can make the result ineffective.
Because skin is elastic in nature, the interface that is established between the injection tube of a needleless injector/syringe and the skin of a patient during the injection of a fluid medicament is of extreme importance. Specifically, several interacting factors must be simultaneously considered. These factors include: seal pressure; holding force; skin stabilization; and tissue damage.
Seal pressure is important because a reliable skin-to-syringe relationship needs to be established which will prevent a leaking of the fluid medicament during an injection. Somewhat related to this concern for seal pressure is the necessity that there also be sufficient force for holding the syringe against the skin during an injection. Specifically, holding forces are important in that they prevent the syringe from coming off the skin due to hand movement of the operator or due to any momentum that may be generated during operation of the injector/syringe. In addition to the concerns regarding relative movement between the patient and the needleless injector/syringe, there is also an important concern for skin stabilization during an injection. Specifically, due to the elastic nature of skin, it is very important during the delivery of a fluid medicament that the injection tube not move off the hole that is created through the skin during the initial impulse. Finally, when collectively considering the above factors, it is a major design concern that tissue damage be absolutely minimized by eliminating sharp edges and projections on the injector/syringe which can cause skin damage and pain.
In order to address the syringe and skin interface concerns mentioned above, several important considerations for the structural design of an injector/syringe must be taken into account. One way by which a seal pressure can be established for the syringe interface with the skin is by suction. If suction is to be used for this purpose, it is necessary to create a suction (partial vacuum) compartment around the injection tube. It happens that both the seal pressure and the holding force will benefit from a larger suction compartment and a greater partial vacuum in the compartment. On the other hand, too large of a suction compartment, and too much suction in the suction compartment, will cause excessive skin distortion that can adversely affect skin stabilization. Further, too much suction (partial vacuum) force in the suction compartment can pull the skin too tight and cause the tip of the injection tube to press too hard against the skin. This, in turn, can lead to tissue damage such as subdermal hematomas (caused by the rupturing of capillaries) or bleeding (caused by an excessive puncturing of the skin beyond what is required for creating an injection hole in the skin). In summary, high sealing pressures and large holding forces can conflict with the desirable objectives of maximizing skin stabilization and minimizing tissue damage. Particularly, in the initial stages of an injection process when impact between the fluid medicament and the skin first occurs.
In order to initiate an injection process with a needleless injector it is obviously necessary to first create a xe2x80x9cholexe2x80x9d in the skin through which a fluid medicament can be injected. It happens, however, that the creation of an appropriate hole in the skin requires that the initial discharge of fluid medicament be under considerable pressure. As a practical matter, the pressure that is necessary to create such a hole is much higher than the pressure that is subsequently required to inject the fluid medicament through the hole. Additionally, it happens that some injections need to be intra-muscular (e.g. some vaccinations), others need to be subcutaneous, (e.g. flu shots or insulin) while others are preferably intra-dermal (e.g. DNA vaccines and allergy testing). The point of all this is that, although seal pressure, holding force, skin stabilization and tissue damage are ever present concerns for needleless injectors, these factors are influenced by design consequences.
As mentioned above, in the manufacture of a needleless injector/syringe it is desirable to avoid the sharp edges and points which can cut, puncture, or otherwise compress tissue. On the other hand, it is also desirable that the tip of the injection tube through which the fluid medicament is delivered be small. One reason for this is that small tips are able to more effectively concentrate forces and establish fluid seals that are sufficient to prevent leaks than are large tips. This, of course, needs to be accomplished without causing tissue compression. Further, due to their more efficient fluid seals, small tips allow for the use of less fluid injection pressures and, thus, slower delivery times can be achieved. The result is a lower cost needleless injector/syringe that has thinner walls with less bulk and is, therefore, less cumbersome to use. In any event, this can not be achieved without properly balancing all of the other factors mentioned above, namely: seal pressure, holding force, skin stabilization and tissue damage.
In light of the above it is an object of the present invention to provide an injector for subcutaneous delivery of a fluid medicament to a patient which is able to slowly inject fluid medicament while avoiding tissue compression or damage. Another object of the present invention is to provide an injector for subcutaneous delivery of a fluid medicament to a patient which effectively stabilizes the skin against the injection tube of the injector during the delivery of the fluid medicament. Still another object of the present invention is to provide an injector for subcutaneous delivery of a fluid medicament to a patient which generates a sufficient seal pressure and a sufficient holding force to prevent the injector from coming off the patient during the delivery of the fluid medicament, and to thereby prevent unwanted lacerations and leaking of fluid medicament. Another object of the present invention is to provide a jet injector which can be configured for use in either intra-muscular, subcutaneous or intra-dermal injections. Still another object of the present invention is to provide a jet injector that has effective pressure variations for accomplishing both the creation of a xe2x80x9chole,xe2x80x9d and the subsequent injection of a fluid medicament through the hole. Yet another object of the present invention is to provide an injector for subcutaneous delivery of a fluid medicament to a patient which is simple to use, relatively easy to manufacture and comparatively cost effective.
In accordance with the present invention, an injector for subcutaneous delivery of a fluid medicament to a patient includes a substantially cylindrical shaped member that has a closed end and an open end. The closed end is defined by a surface, and this surface effectively functions as a base member for the injector. The sides of the cylindrical member are established by an outer wall which extends through a distance between the closed end and the open end. A compartment is thus established. Specifically, the compartment is bounded by the surface of the base member (i.e. the closed end of the cylindrical member), the outer wall which extends from the base member between the closed end and the open end, and the rim of the outer wall (i.e. the open end of the cylindrical member).
Inside the compartment of the injector, an injection tube extends outwardly toward the open end of the cylindrical member from the surface that defines the closed end of the member. Further, an inner wall also extends outwardly from the surface and into the compartment toward the open end. Thus, the compartment is separated into an outer compartment which is located between the outer wall and the inner wall, and an inner compartment that is located between the inner wall and the injection tube. Preferably, the outer wall, the inner wall and the injection tube are concentric. For intra-dermal (shallow) applications, the outer wall, inner wall and injection tube all extend through substantially the same distance from the surface at the closed end of the compartment. For intra-muscular or subcutaneous (deeper) applications, however, it is preferable for the injection tube to extend beyond the outer wall, and for the outer wall to extend beyond the inner wall.
The injector of the present invention also includes a suction means which is connected in fluid communication with both the inner compartment and the outer compartment. As intended for the present invention, when the open end of the cylindrical member (the rim of the wall) is held against the patient, the suction means is activated to draw skin into the compartments. By drawing skin into the compartments with this partial vacuum, the tip of the injection tube is positioned against the skin of the patient to establish an interface seal therewith. While the skin is thus held, it is stabilized to minimize any movement of skin relative to the tip of the injection tube during an injection of fluid medicament to the patient.
During an injection, the inner wall of the injector effectively acts as a skin suction depth control feature. For intra-muscular or subcutaneous injections, where the fluid is to be injected through the skin and respectively into muscle tissue or fat tissue, the inner wall is recessed from the outer wall and from the injection tube. The result in either case is an increased tension in the skin, and a consequent increase in the penetration depth of the injected fluid. Too much tension, however, can have adverse effects such as xe2x80x9cskin creep,xe2x80x9d which results from the elastic nature of skin. The suction depth control features of the inner wall helps keep this tension within acceptable limits. For intra-dermal injections, where skin creep is not a major concern, the coextensive configuration of the outer wall, inner wall and injection tube will cause a comparative reduction in the tension in the skin, with a consequent decrease in the penetration depth of the injected fluid. Further, for shallower infusions (i.e. intra-dermal applications), it may be more effective if the suction remains on for ten to twenty seconds longer than would otherwise be used for intra-muscular applications.
Another aspect of the injector of the present invention is directed toward a configuration that will establish effective pressure variations during an injection. Specifically, for all applications it is desirable that the injector first create a xe2x80x9cholexe2x80x9d in the skin, and that it then maintain a substantially steady infusion of medicament. As contemplated for the present invention, this can be accomplished by providing an air pocket in the distal portion of the injection tube. The effect of this air pocket will be to produce an initial pressure spike as fluid medicament is ejected from the injection tube. Due to this pressure spike, a xe2x80x9cholexe2x80x9d is made through the skin. A normal infusion at lower pressures can then follow. It happens that, up to a point, the larger the air pocket in the distal portion of the injection tube, the deeper will be the penetration of the fluid medicament. Thus, intra-dermal (shallow) applications will most likely benefit from reductions in the size of the air pocket.
For an alternate embodiment of the present invention, the injector is configured to accept inserts that can be used to vary the skin suction depth control features mentioned above. These inserts can be configured in any of several ways. For example, they may have a plurality of inner walls to establish a respective plurality of annular suction compartments around the injection tube, or they can be configured with only the outer wall and a plurality of ribs which extend radially between the outer wall and the injection tube.
For the operation of the injector of the present invention, once the suction means has stabilized skin in the compartments, a drive means which is connected to the injection tube is activated. Upon activation of the drive means, fluid medicament is expelled through the injection tube to first create a hole in the skin, and to then inject the fluid medicament into the patient through the hole. As an example of an injection duty cycle, with the injector tip stabilized, the drive means may be activated for approximately one to five seconds for an injection of approximately one milliliter of fluid medicament.